Plain or slide bearings can be distinguished from roller bearings by having a bearing surface which slidably engages an element upon relative displacement of the bearing and this element, e.g. relative rotation.
In the past, it has been proposed to provide a plain bearing with a support upon which a layer of a bearing metal is disposed.
Plain bearings can be flat, e.g. when they form a guideway for machine parts, but usually are round, e.g. in the form of a sleeve, and the bearing metal surface can be provided along the interior of this sleeve when a rotating shaft is to be journaled therein or, along an exterior of the sleeve, when the plain bearing is carried by the shaft and runs in a stationary member formed with a bore receiving the sleeve.
Typical of the plain bearings which have been provided heretofore are the plain bearings in which, upon the support or substrate, a bearing metal layer predominantly consisting of lead and tin as the alloying components, is galvanically deposited.
Plain bearings of this type have been used among others, e.g. in internal combustion engine systems in which considerable stress may arise. Indeed, in modern technology, plain bearings for internal combustion engine and automotive vehicle applications, by way of example, must be capable of withstanding a wide range of loadings, speeds and operating temperatures and, indeed, even extreme loads, speeds and temperatures.
Because of the bearing requirements and the severe conditions to which such bearings must be subjected, it has been recognized that a plain or slide bearing composed of a single material will generally not be suitable and hence plain bearings of two or more layers in the form of composites have been utilized.
By comparison with casting, sintering and cladding techniques which have all been utilized to form composite bearing structures, the galvanic or electroplating approach to producing a bearing metal layer upon a support, has a multitude of advantages.
For example, it is possible to make exceptionally thin and uniform alloy coatings with exceptionally fine surface properties. The fabrication of the composite bearing can be carried out with substantially less energy cost than classical casting methods. It is possible to utilize alloys which cannot be made with smelting techniques or can be made only with difficulty by such techniques because of, for example, a significant separation in the melting points of the alloying components. Finally, the galvanic or electroplating approach has the advantage that it allows application of alloys with precise dimensioning so that subsequent machining is not required.
As a consequence, multilayer bearings have been fabricated heretofore on a shell, sleeve or sleeve segment of steel to which a lead-bronze coating is applied by melt deposition, by galvanically depositing on the latter a diffusion barrier of nickel onto which bearing layers of PbSn, PbIn or PbSnCu can be applied.
Steel shells, aluminum alloy, nickel or copper or copper-zinc or coper-tin bonding layers, and lead-tin and lead-indium or lead-tin-copper bearing layers have been provided using galvanic technology.
German Pat. No. 844,664 discloses bearing layers of the following compositions:
______________________________________ 1. Sn 6.0 to 12% Cu or Sb or As 0.5 to 6% Pb balance 2. Sn 10% Cu 3% Pb balance 3. Sn 10% Sb 3% Pb balance 4. Sn 10% As 3% Pb balance ______________________________________
These bearing layers are galvanically applied. They have, however, the disadvantage that their corrosion resistance and their fatigue resistance are relatively low. Experience has shown further than the adhesion of these bearing layers to the base metal is poor so that, even in German Pat. No. 844,664, it has been proposed to provide a silver intermediate layer to improve the adhesion.
In Plating, September 1955, pages 1133 to 1136, Puttmann and Roser have described galvanically deposited lead-tin-antimony coatings with 11% tin and 75% antimony which are deposited upon silver intermediate coatings. To the plating bath, hydroquinones and peptones can be added but without being capable of generating the minimum roughness which is required for high loading plain bearings.
Smart, in U.S. Pat. Nos. 2,373,352 and 2,423,624 describes galvanic indium layers to be especially advantageous with lead-bronze bearings in corrosive lubricating oils.
Plain bearings of lead and indium are in use today but have the disadvantage that in the fabrication the lead and indium must be deposited separately and hence the bearing must be subjected to a heat treatment in which diffusion occurs.
In recent years, the increased use of sulfur-containing lubricating oils in the operation of Diesel engines, for example, has resulted in increasing bearing deterioration because of the sulfuric residue which is formed and which accelerates corrosion. It is, therefore, important that the corrosion resistance of the bearing layer be enhanced.
Apart from good corrosion resistance, other important bearing layer requirements include:
1. Embedding ease or the tendency for the bedding layer to be able to allow penetration and capture of hard particles so that they will not remain in a position in which they can alter the lubricating gap and destroy the bearing surface by grooving the latter so that the oil film is interrupted.
2. Heat resistance and little tendency of heat to increase the wear. The bearing layer must be capable of retaining its strength with minimum fall-off as the temperature increases to the operating temperature and beyond.
3. Fit accommodation. This parameter is a measure of the ability of the surface of the bearing to accommodate itself to any contours or irregularities of the surface of the shaft during the initial phases of operation. While this is a reflection of yieldability of the bearing layer, the excessive softness or any limited cohesiveness will simply result in accelerating wear and will not be a true reflection of fit accommodation.
4. Resistance to attack upon or adhesion to the running surface of the element with which the bearing metal is juxtaposed. This parameter represents a low or zero-tendency for the bearing metal to adhere to or attack the aforementioned running surface.
5. Bond strength. By this parameter we refer to the strength of the bond between the bearing metal and the carrier or substrate, it being clear that the bearing metal must be bonded firmly to the support surface so that there is little tendency for the bearing metal to peel away.
6. Fatigue resistance. Under dynamic loading, the bearing layer must have sufficient fatigue resistance.